The heart is capable of undergoing hypertrophic growth in response to a variety of stimuli.
Hypertrophic growth may occur as the result of physical training such as running or swimming. However, it also occurs as the result of injury or in many forms of heart disease. Hypertrophy is the primary mechanism by which the heart reduces stress on the ventricular wall. When the growth is not accompanied by a concomitant increase in chamber size, this is called concentric hypertrophy. Hypertrophy occurs as the result of an increase in protein synthesis and in the size and organization of sarcomeres within individual myocytes. For a more thorough review of cardiac remodeling and hypertrophy, see Kehat (2010) and Hill (2008), each herein incorporated by reference in their entirety. The prevailing view is that cardiac hypertrophy plays a major role in heart failure. Traditional routes of treating heart failure include afterload reduction, blockage of beta-adrenergic receptors (β-ARs) and use of mechanical support devices in afflicted patients. However, the art is in need of additional mechanisms of preventing or treating cardiac hypertrophy.
AKAPs and Cardiac Hypertrophy
Ventricular myocyte hypertrophy is the primary compensatory mechanism whereby the myocardium reduces ventricular wall tension when submitted to stress because of myocardial infarction, hypertension, and congenital heart disease or neurohumoral activation. It is associated with a nonmitotic growth of cardiomyocytes, increased myofibrillar organization, and upregulation of specific subsets of “fetal” genes that are normally expressed during embryonic life (Frey 2004, Hill 2008). The concomitant aberrant cardiac contractility, Ca2+ handling, and myocardial energetics are associated with maladaptive changes that include interstitial fibrosis and cardiomyocyte death and increase the risk of developing heart failure and malignant arrhythmia (Cappola 2008, Hill 2008). Increased in prevalence by risk factors such as smoking and obesity, heart failure is a syndrome that affects about six million Americans and has an annual incidence of 1% of senior citizens (Roger 2011). Since the five-year survival rate after diagnosis is still very poor (lower than 50%), many efforts have been made during the last years to define the molecular mechanisms involved in this pathological process.
Cardiac hypertrophy can be induced by a variety of neuro-humoral, paracrine, and autocrine stimuli, which activate several receptor families including G protein-coupled receptors, cytokine receptors, and growth factor tyrosine kinase receptors (Brown 2006, Frey 2004). In this context, it is becoming increasingly clear that AKAPs can assemble multiprotein complexes that integrate hypertrophic pathways emanating from these receptors. In particular, recent studies have now identified anchoring proteins including mAKAP and AKAP-Lbc and D-AKAP1 that play a central role in organizing and modulating hypertrophic pathways activated by stress signals.
mAKAP. In cardiomyocytes, mAKAPβ is localized to the nuclear envelope through an interaction with nesprin-1α (Pare 2007). mAKAPβ assembles a large signaling complex that integrates hypertrophic signals initiated by α1-adrenergic receptors (α1-ARs) and β-ARs, endothelin-1 receptors, and gp130/leukemia inhibitor factor receptors (FIG. 46A) (Dodge-Kafka 2005, Pare 2005). Over the last few years, the molecular mechanisms as well as the signaling pathways whereby mAKAPβ mediates cardiomyocyte hypertrophy have been extensively investigated. It is now demonstrated that mAKAPβ can recruit the phosphatase calcineurin Aβ (CaNAβ) as well as the hypertrophic transcription factor nuclear factor of activated T cells c3 (NFATc3) (Li 2010). In response to adrenergic receptor activation, anchored CaNAβ dephosphorylates and activates NFATc3, which promotes the transcription of hypertrophic genes (FIG. 46A) (Li 2010). The molecular mechanisms controlling the activation of the pool of CaNAβ bound to the mAKAPβ complex are currently not completely understood but seem to require mobilization of local Ca2+ stores. In this context, it has been shown that mAKAP favors PKA-induced phosphorylation of RyR2 (Kapiloff 2001), which, through the modulation of perinuclear Ca2+ release, could activate CaNAβ (FIG. 46A). In line with this hypothesis, the deletion of the PKA anchoring domain from mAKAPβ has been shown to suppress the mAKAP-mediated hypertrophic response (Pare 2005). On the other hand, recent findings indicate that mAKAPβ also binds phospholipase Cε(PLCε) and that disruption of endogenous mAKAPβ-PLCε complexes in rat neonatal ventricular myocytes inhibits endothelin 1-induced hypertrophy (Zhang 2011). This suggests that the anchoring of PLCε to the nuclear envelope by mAKAPβ controls hypertrophic remodeling. Therefore, it is also plausible that at the nuclear envelope, PLCε might promote the generation of inositol 1,4,5-trisphosphate, which through the mobilization of local Ca2+ stores, might promote the activation of CaNAβ and NFATc3 bound to mAKAPβ (FIG. 46A).
In cardiomyocytes, the dynamics of PKA activation within the mAKAP complex are tightly regulated by AC5 (Kapiloff 2009) and the PDE4D3 (Dodge-Kafka 2005, Dodge 2001) that are directly bound to the anchoring protein. The mAKAP-bound AC5 and upstream β-AR may be localized within transverse tubules adjacent to the nuclear envelope (Escobar 2011). In response to elevated cAMP levels, mAKAP-bound PKA phosphorylates both AC5 and PDE4D3 (Dodge-Kafka 2005, Dodge 2001, Kapiloff 2009). This induces AC5 deactivation and PDE4D3 activation, which locally decreases cAMP concentration and induces deactivation of anchored PKA (FIG. 46A). Dephosphorylation of PDE4D3 is mediated by the phosphatase PP2A that is also associated with mAKAPβ (FIG. 46A) (Dodge-Kafka 2010). Collectively, these findings suggest that the mAKAP complex generates cyclic pulses of PKA activity, a hypothesis that was supported experimentally by live cell imaging studies (Dodge-Kafka 2005).
AKAPs and Hypoxia
Myocardial oxygen levels need to be maintained within narrow levels to sustain cardiac function. During ischemic insult, in response to conditions of reduced oxygen supply (termed hypoxia), cardiomyocytes mobilize hypoxia-inducible factor 1α (HIF-1α), a transcription factor that promotes a wide range of cellular responses necessary to adapt to reduced oxygen (Semenza 2007). Transcriptional responses activated by HIF-1α control cell survival, oxygen transport, energy metabolism, and angiogenesis (Semenza 2007). Under normoxic conditions, HIF-1α is hydroxylated on two specific proline residues by the prolyl hydroxylase domain proteins (PHDs) and subsequently recognized and ubiquitinated by the von Hippel-Lindau protein (Jaakkola 2001, Maxwell 1999). Ubiquitinated HIF-1α is targeted to the proteasome for degradation. On the other hand, when oxygen concentration falls, the enzymatic activity of PHD proteins is inhibited. Moreover, PHD proteins are ubiquitinated by an E3 ligase named “seven in absentia homolog 2 (Siah2)” and targeted for proteasomal degradation (Nakayama 2004). This inhibits HIF-1α degradation and allows the protein to accumulate in the nucleus where it promotes gene transcription required for the adaptive re-sponse to hypoxia. In line with this finding, the delivery of exogenous HIF-1α improves heart function after myocardial infarction (Shyu 2002), whereas cardiac overexpression of HIF-1α reduces infarct size and favors the formation of capillaries (Kido 2005).
Recent findings indicate that mAKAP assembles a signaling complex containing HIF-1α, PHD, von Hippel-Lindau protein, and Siah2 (Wong 2008). This positions HIF-1α in proximity of its upstream regulators as well as to its site of action inside the nucleus. In this configuration, under normoxic conditions, negative regulators associated with the mAKAP complex favor HIF-1α degradation (Wong 2008). On the other hand, during hypoxia, the activation of Siah2 within the mAKAP complex promotes HIF-1α stabilization, allowing the transcription factor to induce transcription (Wong 2008). Therefore, mAKAP assembles a macromolecular complex that can favor degradation or stabilization of HIF-1α in cardiomyocytes in response to variations of oxygen concentrations. In this context, mAKAP could play an important role in cardiomyocyte protection during cardiac ischemia, when coronary blood flow is reduced or interrupted. By coordinating the molecular pathways that control HIF-1α stabilization in cardiomyocytes, mAKAP might favor HIF-1α-mediated transcriptional responses, controlling the induction of glycolysis (which maximizes ATP production under hypoxic conditions), the efficiency of mitochondrial respiration, and cell survival during ischemia (Semenza 2009).
Myofibrillar assembly driving nonmitotic growth of the cardiac myocyte is the major response of the heart to increased workload (Kehay 2010). Although myocyte hypertrophy per se may be compensatory, in diseases such as hypertension and myocardial infarction, activation of the hypertrophic signaling network also results in altered gene expression (“fetal”) and increased cellular apoptosis and interstitial fibrosis, such that left ventricular hypertrophy is a major risk factor for heart failure. Current therapy for pathologic hypertrophy is generally limited to the broad downregulation of signaling pathways through the inhibition of upstream cell membrane receptors and ion channels (McKinsey 2007). Novel drug targets may be revealed through the identification of signaling enzymes that regulate distinct pathways within the hypertrophic signaling network because of isoform specificity or association with unique multimolecular signaling complexes.
p90 ribosomal S6 kinases (RSK) are pleiotropic extracellular signal-regulated kinase (ERK) effectors with activity that is increased in myocytes by most hypertrophic stimuli (Anjum 2008, Sadoshima 2005, Kodama 2000). In addition, increased RSK activity has been detected in explanted hearts from patients with end-stage dilated cardiomyopathy (Takeishi 2002). There are 4 mammalian RSK family members that are ubiquitously expressed and that overlap in substrate specificity (Anjum 2008). RSKs are unusual in that they contain 2 catalytic domains, N-terminal kinase domain and C-terminal kinase domain (FIG. 4A, Anjum 2008). The N-terminal kinase domain phosphorylates RSK substrates and is activated by sequential phosphorylation of the C-terminal kinase domain and N-terminal kinase domain by ERK (ERK1, ERK2, or ERK5) and 3′-phosphoinositide-dependent kinase 1 (PDK1), respectively (Anjum 2008).
By binding scaffold proteins, RSKs may be differentially localized within subcellular compartments, conferring isoform-specific signaling bound to the scaffold protein muscle A-kinase anchoring protein (mAKAP) (Michael 2005). PDK1 activation of RSK was enhanced by co-expression with the mAKAP scaffold in a recombinant system. In cardiac myocytes, mAKAPβ (the alternatively spliced form expressed in muscle cells) organizes signalosomes that transduce cAMP, mitogen-activated protein kinase, Ca2+, and hypoxic signaling by binding a diverse set of enzymes, ion channels, and transcription factors (Kritzer 2012).